"The body cannot heal, because it cannot MAKE ITSELF SICK. It NEEDS no healing. Its health or sickness depends ENTIRELY on how the mind perceives it, and the purpose that the mind would use it FOR."
So. The body itself cannot create its own sickness. The belief that it can or has, is level confusion and a belief in magic. It means you see the body as "causing the mind" instead of the mind causing the body.
So the body cannot "get sick" by itself. It cannot become sick as a result of external things happening. It cannot contract sickness from others. It cannot be attacked without your consent. And it cannot die unless your MIND chooses that it die.
"You (as body) cannot die unless you choose to do so."
So the body cannot become sick "on its own", or as the result of anything other than your mind's decision. It also cannot HEAL ITSELF. The idea that the body has an immune system is an ILLUSION. The body does not heal itself, just as it does not make itself sick.
It is up to the mind to decide, whether or not to make the body sick, or to make the body healthy. The mind can ADD sickness to the body at any time, by free will. The mind can REVOKE sickness from the body at any time, by free will.
"You made sickness and death and therefore can abolish both."
It does not matter what state the body is in. It does not matter if has been sick for 50 years. It does not matter how severe the sickness is or how it was contracted or developed or all the reasons why it is so unchangeable. If the mind chooses health, the body MUST BE HEALED. And when the mind is healthy in its outlook, it WILL see the body in a healthy way and thus FORCE the body to heal "against its will", because it has no will and no way to resist the mind.
"The mind MUST be stronger than the body. Every miracle DEMONSTRATES this."
So no matter what state the body is in or the "story" behind how it seemed to get that way, which is a LIE, the mind CAN heal it at any time. And when it chooses to do so, NOTHING in all the universe has the power to stop it. Even death can be reversed. And when the mind does elect to heal and be healed, it WILL heal the body without exception. "The mind, being sane, heals the body." Such a healing is a MIRACLE. And such miracles can RAISE THE DEAD and reverse EVERY form of bodily sickness.
This is a Course in MIRACLES.